Well, I hold out hope that *something* will happen, eventually.
The Kim's do hold power based on two things, intimidation and information. They control information flow in and out and intimidate their way though rebellion. However, their grip on information is starting to crumble and the fabric of their control over information is fraying around the edges so the Kim's have to step up the intimidation part of the game which they still control. Eventually there won't be any intimidation left to ramp up and the information part will play out.
You are right that the military is the key because it is a double sided blade. Kim is walking a fine line and should the officers start to get tainted by the information that is more and more freely flowing in the country, he will be forced to eliminate the upper ranks at a quicker and quicker rate to stay ahead of it. The harder he presses, the more likely he is to force an armed rebellion.
Kim is caught between a rock and a hard place, he must maintain the appearance of absolute control in the face of mounting internal pressure for change and free market reforms which are already (illegally) taking place inside his country in some places. He can start ramping up the killing, but that will eventually be his undoing. Once the country tips and Kim is removed from power, the resulting violent struggle will happen quickly, but the problem is, nobody will really be able to tell you exactly when the tip will take place.
I think your defector is likely correct in that the *current* conditions are not yet ripe for this tip and Kim will stay in power for the foreseeable future. However, I think change will eventually come to NK though internal forces, barring it being forced into change though external forces. Will it be a decade, two or three? I don't know, but it's pretty clear that change is coming to NK and that change will likely happen while the current Kim is alive..